1. Field Of The Invention
This invention relates to a method and apparatus for powering a pump for recirculation of a regenerated fluid, and more particularly to such a method and apparatus in which the recirculated fluid is utilized to provide at least a portion of the energy requirements for powering the pump.
2. Prior Art
A gas-liquid contactor vessel is commonly utilized for removing desirable or undesirable elements or components from gas mixtures such as removing water vapor from natural gas by contact of the natural gas with a liquid absorber. Gas-liquid contactor vessels may also be used in amine-type sour gas treating, absorption type hydrocarbon liquid recovery plants, and other processes. An amine type treater removes hydrogen sulfide and carbon dioxide from natural gas. A circulating amine solvent may be utilized as the circulating fluid to remove sulfur compounds from the gas stream.
For the removal of water vapor from flowing natural gas, it is desirable to remove the water or water vapor from the natural gas before the gas flows into a gas transmission line. It has been found that polyethylene glycol (commonly triethylene glycol) effectively removes moisture from natural gas when placed in intimate contact with the gas. This is generally done in a pressure (contactor) vessel operating at substantially the flowing pressure of the natural gas, but this requires that the glycol which is recycled from the contactor through a reboiler be injected into the contactor at a pressure slightly higher than the pressure of the natural gas flowing in the contactor.
Heretofore it has been the practice to use the driving force of the moisture laden rich glycol to operate a fluid motor and associated pump for pumping regenerated lean glycol back into the pressurized contacting vessel. For example, U.S. Pat. No. 4,511,378 shows an energy exchange apparatus wherein a gear motor is driven by wet glycol and gas to operate a gear pump for pumping regenerated dry glycol. In this apparatus, the volume displacement of the gear motor is sufficiently greater than the volume displacement of the gear pump so as to provide adequate power to overcome friction, head, piping and other losses in the system in addition to powering the pump.
Another example is found in U.S. Pat. No. 4,427,420 in which an electric motor drives a separate pump for providing supplemental glycol circulation from the reboiler to the pressurized contacting vessel. Each of these arrangements has its particular drawbacks; in the wet glycol of the '378 patent, the motor volume must be greater than the pump volume and the extra volume of the motor means that gas is used and must be expelled to the atmosphere or reclaimed by a low pressure gathering system and recompressed. Some of this gas can be burned in the reboiler, but most users prefer not to burn wet gas in the reboiler because it corrodes the burner elements, fouls the fire tubes, and is not at all reliable due to entrained liquids.
The system utilized in the '420 patent utilizes two separate pumps independent of each other with each pump receiving only a portion of the dry glycol from the regenerator. One of the pumps is driven by a fluid motor and the other pump is driven by an electric motor to maintain volumetric balance in the system. While the '420 patent does not vent natural gas to atmosphere, many of the recirculating systems being used vent a significant portion of natural gas to the atmosphere which results in environmental degradation as well as wasting valuable energy in the flared gas. Also, the '420 patent must pump all the volume required by the reciprocating piston pump due to its imbalance. This volume must be supplied at contactor pressure so an inordinate amount of electric power must be used.
The present invention provides a new and improved apparatus and method which retains the principal benefits of the earlier energy exchange unit by recovering substantial energy from the system by using as much of the energy as is available from a given volume of liquid in the system before the liquid pressure is reduced to near atmosphere pressure of the reboiler to drive a recirculating pump. It avoids the disadvantages of the earlier system described above in the '420 patent which requires a separate electric motor and separate pump to complement the energy exchange drive powered only by the recirculating liquid. The only electric power required in the present invention is to overcome the slight friction encountered in the pump--motor friction and associated hydraulic friction in the piping. Aside from the obvious environmental advantage of reducing emissions, the present invention also avoids the economic waste inherent in a system that vents gas into the atmosphere because that cost, at present day gas prices, far exceeds the cost of electricity required to operate a small electric motor.
An object of the present invention is to provide a method and apparatus for the removal of selected elements from a gas mixture by providing a pump for recirculation of a removal fluid through a gas-liquid contactor vessel.
A further object of the present invention is to provide such a method and apparatus for the removal of selected elements from natural gas while utilizing the recirculating removal fluid to maximize the energy requirements for powering a pump to pump the regenerated removal fluid into the gas-liquid contactor vessel.
An additional object is to provide an auxiliary drive means operably connected to the pump to supplement adequately the power provided to the pump from a fluid motor driven by the removal fluid.